To create different planetary systems I had to undergo austerities and penance, and the Lord, thus being pleased with me, incarnated in four sanas [Sanaka, Sanatkumāra, Sanandana and Sanātana]. In the previous creation the spiritual truth was devastated, but the four sanas explained it so nicely that the truth at once became clearly perceived by the sages.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तप्तं तपो विविधलोकसिसृक्षया मे आदौ सनात्स्वतपसः स चतुःसनोऽभूत् । प्राक्कल्पसम्प्लवविनष्टमिहात्मतत्त्वं सम्यग्जगाद मुनयो यदचक्षतात्मन्
taptaṃ tapo vividhalokasisṛkṣayā me ādau sanātsvatapasaḥ sa catuḥsano'bhūt | prākkalpasamplavavinaṣṭamihātmatattvaṃ samyagjagāda munayo yadacakṣatātman
SB 2.7.5